For the preparation of an amorphous or polycrystalline functional film such as semiconducting film, insulative film, photoconductive film, magnetic film or metallic film for use in thin-film transistor (TFT) or the like, there have been proposed a number of methods suited for the respective films from the viewpoints of expected physical characteristics thereon and their uses.
For instance, for the preparation of a deposited silicon-containing film, namely, a non-monocrystalline silicon film of which unpaired electrons are compensated with a compensating agent such as hydrogen atom (H) or halogen atom (X) [hereinafter referred to as "NON-Si(H,X)"] such as an amorphous silicon film of which unpaired electrons are compensated with said compensating agent [hereinafter referred to as "a-Si(H,X)"] or polycrystalline silicon film of which unpaired electrons are compensated with said compensating agent [hereinafter referred to as "poly-Si(H,X)"], wherein the so-called crystallite silicon film lies in the range of the aforesaid a-Si(H,X) as a matter of course, there have been proposed various methods using vacuum evaporation technique, heat chemical vapor deposition technique, plasma chemical vapor deposition technique, reactive sputtering technique, ion plating technique and light chemical vapor deposition technique.
Among those methods, the method using plasma vapor deposition technique (hereinafter referred to as "plasma CVD method") has been generally recognized as being the most preferred and is currently used on a commercial basis.
However, the operation conditions to be employed under the plasma CVD method are much more complicated than the known CVD method, and it is extremely difficult to generalize them.
That is, there already exist a number of variations even in the correlated parameters concerning the temperature of a substrate, the amount and the flow rate of gases to be introduced, the degree of pressure and the high frequency power for forming a film, the structure of an electrode, the structure of a reaction chamber, the flow rate of exhaust gases and the plasma generation system. Besides said parameters, there also exist other kinds of parameters. Under these circumstances, in order to obtain a desirable deposited film product it is required to choose precise parameters from a great number of varied parameters. Sometimes, serious problems occur. Because of the precisely chosen parameters, a plasma is apt to be in an unstable state. This condition often invites problems in a deposited film to be formed.
In addition, since a plasma is directly generated by the action of a high-frequency wave or a microwave in a film forming space wherein a substrate is placed, electrons or ion species resulted therein sometimes come to damage a film to be deposited on the substrate. In that case, the resulting film product eventually becomes such that has an undesired quality unevenness and quality deterioration.
And for the apparatus in which the process using the plasma CVD method is practiced, its structure will eventually become complicated since the parameters to be employed are precisely chosen as stated above.
Whenever the scale or the kind of the apparatus to be used is modified or changed, the apparatus must be so structured as to cope with the precisely chosen parameters.
In this regard, even if a desirable deposited film should be fortuitously mass-produced, the film product becomes unavoidably costly because (1) a heavy investment is firstly necessitated to set up a particularly appropriate apparatus therefor; (2) a number of process operation parameters even for such apparatus still exist and the relevant parameters for the mass-production of such film. In accordance with such precisely chosen parameters, the process must then be carefully practiced.
As a solution to solve those problems as mentioned above, the so-called indirect plasma CVD method has been proposed.
The indirect plasma CVD method comprises generating plasmas in an upper stream space apart from a film forming space with microwave energy or the like and transporting the plasmas in the film forming space having a substrate so that chemical species are selectively utilized to form a deposited film on the substrate.
However, in this indirect plasma CVD method, as a plasma is generated not in the film forming space but in a different space apart therefrom and the plasma is then transported in the film forming space, the life span of the chemical species must be long enough to form a deposited film. In this connection, for this indirect plasma CVD method, there exist problems. That is, firstly, only a limited kind of a deposited film can be obtained because the kind of a starting gas to be employed is limited to such that gives a chemical species of a long life span. Secondly, a large quantity of energy is necessitated for the generation of plasmas in the aforesaid space. Thirdly, the generation of a chemical species capable of contributing in forming a deposited film and which has a sufficiently long enough life span can not be easily attained. Finally, it is difficult to maintain the original amount of such chemical species until the formation of a deposited film.
Besides this indirect plasma CVD method, there has been attempted to use a method using light chemical vapor deposition technique (hereinafter referred to as "light CVD method").
For the light CVD method, although there is an advantage that neither such electron nor such ion species as giving a damage to a film to be deposited on a substrate does not occur, there exist problems.
That is, firstly, only a limited number of light sources can be practically used. Secondly, even for such light source, its wavelength has a partiality toward the ultraviolet side, therefore a large scale light source and a particular power source are necessitated.
Thirdly, since the inner surface of a light transmission window gradually becomes coated with a film following the proceeding of the film forming process, the quantity of light to be incidented through the transmission window into a film forming space eventually becomes decreased to thereby retard the film deposition on the substrate.
Against this background, there is now an increased demand for a method that makes it possible to practice the process at a high film-forming rate in a simple procedure in order to mass-produce a desirable semiconducting film for thin-film transistor having a relevant uniformity and having many practically applicable characteristics and such that the product will be relatively inexpensive.
Likewise, there is a similar situation which exists with respect to other kinds of semiconducting films for thin-film transistor, such as silicon:nitrogen film, silicon:carbon film and silicon:oxygen film.